Hydrofluoric acid process



June 26, 1945. M. p MATUSZAK 2,379,022

HYDROFLUORIG ACID PROCESS Filed May 15, 1943 INVENTOR MP. MATUSZAK BY .gg am,

ATTOR EYS Patented June 26, 1945 RoFLUoRIo Aem raoonss Maryan P. Matuszak, Bartlesville, Okla., assigner to Phillips Petroleum Company, a corporation of Delaware application May i5, 1943, serial No. 487,143 -s cia. (ci. 26o-esac) This invention relates to a process of purification of hydroiiuoric acid, more particularly to the removal of sulfur dioxide.

Hydrouoric acid, especially in a concentrated or anhydrous form, is an exceedingly useful catalyst for e'ecting various conversions of organic compounds, among which are alkylation of all.inflatable organic compounds, especially aromatic hydrocarbon and non-hydrocarbon compounds and of paraiilnic hydrocarbons, with olens or other alkylating reactants, such as alcohols, esters, alkyl halides, ethers, etc., polymerization of olens, and reconstruction, isomerization,

.and/or disproportionation of hydrocarbons. As

commercially prepared by the action oi' concentrated sulfuric acid'on uorspar or fluorite or similar minerals, however, it always contains a small proportion of sulfur dioxide, which in the best grade of commercial anhydrous hydrofluoric acid usually is not more than 0.1 per cent by weight, but sometimes is of the order of 0.5 to 1 per cent or more. The presence of this sulfur d1'.- oxide is troublesome and deleterious, ysince the sulfur becomes concentratedl in the product of the catalytic conversion, thereby increasing the corrosiveness of the product, decreasing the octane number and/or lead response of the product (if a motor fuel), and necessitating a considerable expense for desulfurization of the product. For example, it has been repeatedly observed that the product of the alkylation of isobutane with oleflns in the presence of a fresh batch of hydrouoric acid is sour to doctor solution and at times contains enough sulfur to liberate readily detectable proportions of hydrogen sulfide, or evenelementary sulfur, when it is distilled, in spite of the fact that comparatively negligible orno sulfur was present in the original hydrocarbon feed stocks. The appearance of excessively high' proportions of sulfur in the alkylate has been traced to sulfur dioxide present as an impurity in the hydrofluoric acid used as catalyst.

An object of this invention is to purify hydrouoric acid by substantially complete removing of sulfur dioxide.

Another object of thisinvention is to remove Sulfur dioxide from hydrouoric acid by distilla tion, preferably in the presence of an added parailin.

Another object is to obtain the advantages of a completely sulfur dioxide-free hydrofluoric acid as a catalyst for the conversion of hydrocarbons.

Other objects and advantages of this invention will be apparent from the a companying description and discussion.

In accordance with one embodiment of this invention, highly concentrated or substantially anhydrous hydrouoric acid is subjected to a fractional distillation that removes all traces of sul-- fur dioxide. Simple fractional' distillation in the absence of any kadded substance may be used, but such simple fractionalA distillation is relatively time-consuming and relatively inefficient for the removal of the last traces of sulfur dioxide, and

at times it results in the loss of considerable hydrouoric acid that is distilled on with the sulfur dioxide. Hence it is advantageous to eiect the distillation in the presence of an added lowboiling parain, such as a parailin of three to ve carbon atoms, which forms a minimum-boiling azeotropic mixture with the sulfur dioxide and which has the eiect oi relatively increasing the efficiency of the fractional distillation. Whenever possible, the added parailin is preferably one that is subjected to catalytic conversion in the presence of the hydrcuoric acid in a subsequent conversion step, so that, when an excess of the paramn is used, the excessiparain need not be removed but instead is carried with the purified hydroiluorlc acid to the conversion step. When the conversion step is one of alkylation of a lowboiling alkylatable parn, such as isobutane and/or isopentane, the addition cf an excess of this paraihn results in presaturation of the hydroiiuoric acid alkylation catalyst with the paraifln, in a manner equivalent to that disclosed in my copending application Serial No. 404,395, led July 28, 1941, of which the present application is considered-to be a continuation in part insofar as any common subject matter is concerned. However, in the broad scope of this invention, the resulting sulfur dioxide-free hydrouoric acid is removable as a product of the purication step herein disclosed, and it may be subsequently used for any purpose whatsoever, as will be understood by those skilled in the art of utilizing hydroiluoric acid. By the practice of my invention it` is possible to remove the sulfur dioxide so that it cannot be detected in the resulting purified hydrouoric acid by ordinary analytical methods,

that is, to a concentration not greater than about' 0.0001 per cent by weight of the hydrofluoric acid. An understanding of some aspects of the present invention may be facilitated by the accome one arrangement of alkylatable hydrocarbon in the presence of the resulting purified hydroiluoric acid.

Commercial anhydrous hydrofiuoric acid, containing about 0.1 per cent sulfur dioxide by weight, enters fractionating column I through inlet II controlled by valve I2. A low-boiling parailn having three to ve carbon atoms per molecule is preferably added, as through inlet I3 controlled by valve Il. For the sake of concreteness and simplicity, this parailin may be taken to be isobutane, which is a low-boiling isoparafiin that is readily alkylatable to motor-fuel hydrocarbons.

By fractional distillation in column I0, the hydroiiuoric acid is freed from the sulfur dioxide, which is withdrawn overhead from the column, when no added parafiin is present, through outlet I5 controlled by valve I6. -When an added parafiin, such as isobutane, is present the sulfur dioxide is removed as a relatively low-boiling or a minimum-boiling azeotropic mixture with the added paramn. The composition of the azeotropic mixture varies somewhat with the conditions of operation of column I0. For example, when the distillation is performed under substantially equilibrium conditions, the content of sulfur dioxide in the azeotropic mixture increases from about 55 mol per cent at about 0 C. to about 65 mol per cent at about 100 C. Since distillation under equilibrium conditions is not necessary, ordinarily the content of sulfur dioxide generally is somewhat less than these values; and the distillation is advantageously carried on more rapidly'than would be possible if the mixture going overhead were limited to the exact composition of the azeotropic mixture for the temperature at the top of column I0. In such distillation, the overhead effluent from column I0 at times contains some hydroiluoric acid, since hydrofluoric acid also forms low-boiling or minimum-boiling azeotropic mixtures with paramns having three to nve carbon atoms per molecule. However, since the parailin-hydrofluoric acid azeotropes are higher--- boiling than the parailin-sulfur dioxide azeotropes, and since any vaporized hydrofluoric acid is positively diluted by the paraffin, the proportion of hydrofluoric acid in the overhead eiiiuent from column I0 is much less than that characteristic of distillation in the absence of an added parafiin; consequently, the loss of hydrofluoric acid in the overhead eiiluent is advantageously decreased when addition of a paraflin is practiced.

Although the resulting paraflln-sulfur dioxide overhead eilluent may be withdrawn through outlet I5, it is usually preferably passed through conduit I1 controlled by valve I8 to separator 20, in which it is cooled sufficiently to condense and separate it into two liquid phases under the inuence of gravity and/or centrifugation.v The temperature of the mixture in separator 20 must be below the critical solution temperature, and the separation is improved as the temperature isv decreased below the critical solution temperature. The optimum temperature decreases with decrease in the molecular weight of the added paraflln, being in the range of approximately to 30 C. for a butane and being about 10 C. lower and higher forpropane and pentane, respectively; it may be readily determined for any. particular instance by trial. The lower or denser phase in separator is primarily sulfur dioxide having in solution only a minor proportion of added parailln and possibly a trace or generally negligible proportion of hyper orlighter phase in separator 20 is primarily added parafiln having in solution only a. minor proportion of sulfur dioxide and a smaller proportion of hydrofluorie acid; it is returned as reux to column I0 by pump 23 through conduit 24 controlled by valve 25.

If desired, the proportion of paraflin added to column I0 may be so adjusted that pure hydroiiuoric acid, free from -both sulfur dioxide and added parain, is withdrawn as a kettle product to storage through outlet 26 controlled by valve 21. When some paraiiin material is not deleterious to the hydrofiuoric acid, as when the added paraii'in is one taking part in a. subsequent conversion step, however, it is preferable to add an excess of the parafiin, so that the hydorfluoric acid leaving column I0 is substantially saturated with dissolved paraffin. The maximum proportion of parailin that can be dissolved in the hydrofiuoric acid depends upon the temperature, as is illustrated by the fact that the solubility of liquid isobutane in liquid hydrofluoric acid increases practically linearly with increase in temperature from 1.8 per cent by weight at 0 C. to 4.0 per cent at 60 C. When the sulfur dioxide-freed hydroiluoric acid is'to be used as a catalyst in `the catalytic alkyiation of isobutane, it is passed, together with dissolved isobutane, through conduit 28 controlled by valve 29 to alkylator 30; when its temperature is higher than is .desired for the alkylation, it may be cooled in a, cooler, not shown, be-

fore passing into allaylator 30.

In alkylator 30,-the hydrofluoric acid containing dissolved isobutane is intimately mixed with a hydrocarbon feed suitable for the desired alkylation that enters the alkylator through inlet 3l controlled by valve 32. This feed is commonly a mixture comprising the alkylatable hydrocarbon, such as isobutane, and the alkylation reactant. such as one or more olefins, preferably oleiins of three to iive carbon atoms per molecule, further preferably butylenes, and still further preferably isobutylene; in addition, the feed generally comprises one or more incidentally present and relatively inert hydrocarbons, such as propane and normal butane. The reaction conditions in alkylator 30 are selected with due regard to the optimum economical production of high-quality alkylate. The instantaneous concentration of alkylating reactant should be low, whereas that of the allsyiat-y able hydrocarbon should be high, in order to minimize undesired side reactions. In practice,

. the over-all mol ratio of isobutane entering the drofluoric acid; it is withdrawn from the process through outlet 2| controlled by valve-22. The pprocess and being recycled to butylene is usually between 4:1 and 10:1: it is preferably as high as is economically obtainable. In the reaction zone itself the mol ratio of isobutane to butylene is much higher, preferably well over 1. The volume ratio of hydroiluoric acid to hydrocarbons should be in the range of 0.5:1 to 2:1; it is generally in the neighborhood of 1:1, which is preferred. 'Ihe temperature generally may be in a rather broad range, such as 0 to 70 C., but in the particular embodiment under consideration, it is preferably in the range 01150 to '10 C., to minimize the expense ot cooling the hydroiluoric acid from column I0. Cooling of the reaction zone is usually desirable and may be effected indirectly with water. The pressure is preferably sufllcient to maintainall components of the reaction mixture in the liquid phase, but an unusually high pressure is not necessary. Thereaction time, or time oi' residenceo the reaction mixture in alkylator 30, may be between about 1 and 30 minutes, preferably between 3 and 15 minutes.

the range of 30 to 40 C., or lower, into a hydrofiuoric acid liquid phase and a hydrocarbonlliquid phase. The hydrofluoric acid phase is recycled by pump 4| through conduit 42 controlled by valve d3 to alkylator 30. Usually, at least a part of this phase is preferably passed through conduit 44 controlled by valve d5 .to fractionator 50, wherein it is purified by being distilled from a heavy ac'idsoluble oil, which is Withdrawn through outlet 4G controlled by valve El. The purified hydrofiuoric acid, accompanied by isobutane that was dissolved in the acid, is passed through conduit 48 controlled by valve Q .to alkylator 30.

The hydrocarbon phase separated in separator i0 is pumped by pump 5l through conduit 52 controlled by valve 53 to fractionating means 60, which generally consists of several fractionating columns and appropriate auxiliary equipment as will be understood by Ithose skilled in the art of distilling materials. The hydrocarbon mixture is v Hydrogen fluoride 98.7

Sulfur dioxide 0.070 Fiuosilicic acid 0.28

Sulfuric 'acid 0.008 Water 0.25 Unaccounted for 0.692

Total 100.000

fractionally distilled in means S0 into several fractions, as may be desired, such as the following: (l) a minor fraction, comprising chieiiy light gases having less than four carbon atoms per molecule, which is withdrawn through outlet 55 controlled by valve 56; (2l a fraction, comprising chieiiy isobutane and hydrouoric acid, which is recycled through conduit 5i controlled by valve 58 to alkylator 30; (3) a major fraction, comprising chiefly acid-free isobutane, which is recycled through conduit 5d to fractionating column i0 and/or alkylator in proportions controlled by valves 6i and 02; (4) a fraction, comprising chiey normal butane, which is withdrawn through outlet 63 controlled by valve iid; (5) a motor-fuel fraction, comprising chiei'iy the gasoline-range paramns formed by the alkylation, which is withdrawn as a product through outlet 05 controlled by valve t6; and (6) a minor oil fraction, comprising chiefly hydrocarbons boiling above the gasoline range, which is withdrawn through outlet 0l. controlled by valve 68.

it will be understood that the drawing is schematic and that additional equipment wellknown in the art may be used to facilitate obtaining eects that have been indicated.

Some aspects of the invention are further illustrated by the following example, which should not be used to restrict the invention unduly.

Example Shortly after a commercial plant for the hyq- Propane 10.8 Isobutane 33.9 Eutylene and propylene 5.1 Normal butane 1 50.1 Pentane 0.1

liotal 100.0

This feed contained only 0.000,5 per cent by weight ci sulfur. The residence time of the hydrocarbon phase in the alkylation zone was 1,0 minutes; the reaction temperature was 32 VC. The ratio of hydroiiuoric acid to hydrocarbons in the reactor was approximately 1:1 by volume. The composition of the hydrouoric acid used as catalyst is indicated by thekfollowing typical analysis, in per cent by weight:

In contrast, when the catalyst is preliminarily freed from sulfur dioxide by distillation in the presence of added isobutane, the alkylate is sweet to doctor solution and usually contains less than about 0.0005 per cent of sulfur; and the lead Vresponse of the alkylate is comparatively much better.

The invention may be practiced otherwise than as specifically described or illustrated herein, and many modications within the scope and spirit of it will be obvious to those skilled in the art, in the light of the present disclosure and discussion.

I claim:

l. The process of removing essentially all the sulful dioxide which is associated with concentrated hydrouoric acid as an impurity, which comprises subjecting said impure hydrofluoric acid to distillation in the presence ofy a low-boiling paramn hydrocarbon having at least three and not -more than five carbon atoms per molecule to remove as a low-boiling fraction a mixture of said paraiiin and sulfur dioxide and containing essentially all the sulfur dioxide contained as an impurity inA said hydrouoric acid.

2. A process for removing sulfurl dioxide from anhydrous hydrofiuoric acid containing same as, an impurity in an amount substantially greater than 0.0001 per cent by weight, which comprises -boiling parafiin hydrocarbonl is a butane.

4. The process'of claim 2 in which saidv lowboiling paramn hydrocarbon is a butane.

5. In a process for alkylating a low-boiling isoparain with an olefin in the presence of a hydrofluoric acid alkylation catalyst, in which an unreacted isopraraflin fraction is separated from the alkylation eiliuent and recycled to the alkylation,

' the improvement which comprises passing sulfur dioxide-contaminated hydrouoric acid to a fractional distillation, passing a portion of saidv recycled isoparaiiln fraction to said fractional distillation in an amount at least suicient to form a minimum-boiling azeotropicmixture with all of said sulfur dioxide and to have an excess dissolved in liquid hydrofiuoric acid ,removed from said distillation as a kettle product, removing from said distillation as an overhead product a mixture comprising sulfur dioxide and said isoparaln, removing from said distillation as a kettle product liquid hydrofluoric acid essentially free from sulfur dioxide and containing dissolved isoparatiln, and passing said kettle product to said alkyl'ation as a portion of the alkylation catalyst.

6. A process for alkylating a low-boiling alkylatable paramn hydrocarbon with an alkylating reactant in the presence of concentrated hydrofluoric acid and avoiding contamination of the alkylate by sulfur compounds resulting from sulfur dioxide present as an impurity in makeup hydrouoric acid catalyst, which comprises reacting an alkylatable low-boiling parafiin hydrocarbon and an alkylating reactant under alkylation conditions in the presence of essentially 15 sulfur-free hydroiiuoric acid as alkylation cat- 10 mixture comprising sulfur dioxide and low-boiling parain hydrocarbons. removing as a kettleproduct from saidmeans substantially pure sulfur 'dioxide-free hydrofiuoric acid and passing same to said alkylation.

MARYAN P. MATUSZAK. 

